Waves: Part 9 - Propagation Inversion
As a child I came under two powerful influences. The first of these was electricity. My father was the chief electrician at a chemical works and he would bring home any broken or redundant electrical parts for me to tinker with, dismantle and occasionally repair.
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In my teens the new-fangled transistor was starting to replace the vacuum tube and the most obvious result was the replacement of the traditional table-top tube radio with the dry battery powered transistorized portable radio. This meant that huge numbers of tube radios were surplus to requirements and could be obtained for peanuts. It was not long before I obtained a flagship model tube radio. In all technical aspects, this left the transistor radio in the dust. The sensitivity, selectivity and sound quality were all vastly superior, and the number of stations it could pick up was far greater, a factor that would increase further when I rigged up a decent antenna.
Later I would learn that early transistors had a high noise floor that drowned weak signals. I also learned not to expect any low frequencies out of a tiny loudspeaker or to get much energy out of a battery. Only the first problem has been remedied today.
American readers may find this a mystery, but at one time most European countries operated a state monopoly on broadcasting and communications. In the absence of competition, the state broadcasters had become highly complacent and most of their output was somewhere between propagandist and tedious.
In order to challenge the monopoly in UK, a number of entrepreneurs persuaded the government of Luxembourg, a small state wedged between Germany, Belgium and France, to grant them broadcasting rights. This resulted in Radio Luxembourg that began broadcasting to the UK in the nineteen thirties, much to the indignation of the British authorities, who could do nothing about it.
The Second World War silenced the station, but afterwards it came back on a wavelength of 208 meters (1439kHz), which in UK would be on the medium wave band, known in USA as AM.
It was not long before I found Radio Luxembourg when tuning up and down the multiple wave bands of my radio. I suppose now that would be called surfing, but in those days surfing was what the Beach Boys sang about.
I came across another phenomenon that I didn’t understand. Reception during daylight hours was not very good, whereas just before sunset it improved dramatically and remained good during the hours of darkness.
To see why that happens, we need to go back to Guglielmo Marconi’s early radio experiments. He was told that long range radio communication was impossible because of the curvature of the Earth. Radio waves directed horizontally at the transmitter would go off into space never to return. After all, the moon has a dark side where light from the sun can’t reach it.
Marconi thought differently, because he had already performed experiments that confirmed communication beyond line of sight. The moon has no atmosphere, whereas the Earth does. Marconi discovered that reception was better during darkness, predating my own discovery by a substantial margin. However, like me, he didn’t at first know why.
At roughly the same time, Kennelly in USA and Heaviside in the UK suggested that there must be a layer in the atmosphere that is capable of reflecting radio waves back to Earth, allowing long distance reception. Such layers were found to exist, in which many of the constituent gases were ionized, which led to those layers being called the ionosphere.
Fig.1 shows that one of the layers could be found at 100 to 140km above the Earth, whereas in daylight, the solar wind could penetrate the atmosphere and create a lower layer starting at about 50km. The lower layer would reflect radio waves back closer to the transmitter, hence the improvement found after dark, when the lower layer dispersed.
The existence of these ionized layers and of long-range reception meant that the reflection process was real, but controversy continued about the mechanism of the reflection. The ionized layer was far from being solid like a mirror, so the process that returned the radio signals to Earth had to be one where there was a gradient in the propagation velocity that would turn a wave front downwards.
That idea was met with the objection that radio waves in the atmosphere propagate at very nearly the speed of light in a vacuum, and that for the idea to work the speed of light would have to be exceeded, in violation of the laws of relativity.
Those who have been following this series will recall that I have often distinguished between phase velocity and group velocity in wave propagation. Group velocity is always less than phase velocity. Here’s another distinction: only waves travelling at the group velocity can carry information, and relativity only forbids information travelling faster than light.
That idea allows the phase velocity to exceed the speed of light; provided the group velocity falls by the same proportion. In other words, the ionosphere is dispersive. In a dispersive medium, the only waveform that can travel at phase velocity is a sine wave, which has no bandwidth and so carries no information.
Bouncing swiftly back to Earth, few people will be unaware of the revolution in popular music that took place from the late 1950s/early 1960s. Most state broadcasters took the patronizing view that it was a temporary distraction to their ideas that would soon fade away. Many popular songs were about relationships, but some were protest songs, which also went down badly with the establishment.
Fig.1 At left, during nighttime, the reflecting layer in the atmosphere is high and reflections can travel a long way. At right, during the day a lower layer forms, restricting range.
As is well known, it didn’t fade away, and monopolistic state broadcasters failed to address the demand. It was then realized that states have no jurisdiction over the open sea beyond a certain distance, which was three miles, later extended to twelve miles. It followed that broadcasting from a ship could not be stopped by any national laws.
One of the first examples of offshore radio was Radio Veronica, which from 1960 was anchored off the coast of the Netherlands and broadcast pop music with DJs speaking Dutch. Radio Luxembourg had sold blocks of airtime to record companies and anyone outside that arrangement had difficulty getting their acts played.
That resulted in the birth of Radio Caroline, named after the daughter of President Kennedy, which started broadcasting off the British coast in 1964. The success of Radio Caroline caused a number of other offshore stations to start up, and the idea came to be known as pirate radio. With my tube radio I could receive them all. I made the mistake of mentioning some of this to my rich friends at school who all had transistor radios, and they had no idea what I was talking about. I still have that problem, of course.
This time the monopoly of the BBC was seriously under threat, because more people were listening to Caroline and other pirates than to the BBC. They did what any self respecting monopoly should do, which was to get them closed down. Because the ships were outside territorial limits, no action could be taken directly. Instead it was made an offence to provide any service from land to such ships. This legislation took most of the pirates off the air, whereas Caroline kept going because it could be provisioned from the Netherlands, where there was as yet no equivalent law.
Having disposed of most of the competition, the BBC then had to provide an alternative, when it had made no plans to do so. Their attempt was known as Radio 1 and it was a mess because they had next to no spare frequencies or transmitters. They had next to no DJs and had to hire former pirates. During the pirate heyday one of the complaints levelled against them was that they interfered with legitimate stations. It was therefore ironic that when Radio 1 went on the air, it was found to share the same frequency as Radio Tirana, based in Albania.
Radio Tirana had nothing to say, but it clung to its allotted frequency by repeatedly broadcasting the first bars of the national anthem. This was the time when California in particular was going through the hippy era and all was peace and love and flower children. This was reflected in popular music, with “Let’s go to San Francisco” and “Flowers in the Rain”.
I shall never forget listening to Flowers in the Rain mixed with the Albanian National anthem. It symbolized the end of an era that I had been lucky to live through thanks to my junk shop tube radio. I suspect my subsequent career was influenced heavily by that era, and it nurtured my distrust in authority.
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